In plastic bags in which a tablet or powder drug, for example, is to be contained in closed condition, the presence of any pinhole may lead to a very critical situation. In the case of such plastic bags, therefore, it is necessary to inspect the pinhole strictly.
In a conventional known pinhole inspecting method, a substance which is liable to change in color tone under oxygen or humid conditions, for example, is contained in closed condition in a packaging container, then it is judged on the basis of change in the color tone of the substance whether or not the pinhole or the like is present. Furthermore, a pinhole inspecting method in the case of a rigid container is known in which the container is filled with an inspecting gas under pressure and any leakage gas is inspected by a gas detector to find whether or not a pinhole is present. The gas detector converts any inspecting gas leakage into an electric signal corresponding to amount of the leakage per unit time.
In Japanese Patent Application Nos. 5-344297 and 5-339527, there is disclosed a method and an apparatus, developed by the present inventor, for inspecting a pinhole of a flexible container, such as plastic bag etc., by using a gas detector. In this pinhole inspecting method, any possible leakage of inert gas from an inert gas-filled, closed plastic bag is electrically checked. According to the inspecting method, a threshold value corresponding to a change in electrical conditions caused by environmental change is preliminarily determined for each measurement, and it is judged by checking that the measured value has exceeded the threshold value or otherwise whether or not a gas leakage or a pinhole is present.
In the foregoing pinhole inspecting method, since it is judged whether or not the measured value has exceeded the threshold value, even if a pinhole does exist, it is necessary to wait long until the measured value has reached the threshold value. Therefore, such a problem arises that considerable time is required for quality judgement with respect to containers, and this adversely affects the productivity of the whole manufacturing line.
Furthermore, for example, in the case that the same kind of gas as the gas in the container exists in the ambient atmosphere of the container, namely the object for measurement, the gas will be detected even if a pinhole of the container is not present. Therefore, such a problem arises that it becomes difficult to inspect a pinhole accurately and reliably, thus resulting in drop of reliability of inspection.
An object of the present invention is to provide, with a view to eliminating such conventional problems, a pinhole inspecting apparatus which enables accurate and rapid inspection of pinhole of a closed container so as to satisfy recent requirements of strict quality, performance and rapidity.